Histamine [2-(imidazol-4-yl)ethylamine] is a transmitter substance. Histamine exerts a physiological effect via multiple distinct G-protein coupled receptors. It plays a role in immediate hypersensitivity reactions and is released from mast cells following antigen IgE antibody interaction. The actions of released histamine on the vasculature and smooth muscle system account for the symptoms of the allergic response. These actions occur at the H1 receptor (Ash, A. S. F. and Schild, H. O., Br. J. Pharmacol., 1966, 27, 427) and are blocked by the classical antihistamines (e.g. diphenhydramine). Histamine is also an important regulator of gastric acid secretion through its action on parietal cells. These effects of histamine are mediated via the H2 receptor (Black, J. W., Duncan, W. A. M., Durant, C. J., Ganellin, C. R. and Parsons, E. M., Nature, 1972, 236, 385) and are blocked by H2 receptor antagonists (e.g. cimetidine). The third histamine receptor -H3- was first described as a presynaptic autoreceptor in the central nervous system (CNS) (Arrang, J.-M., Garbarg, M., and Schwartz, J.-C., Nature 1983, 302, 832) controlling the synthesis and release of histamine. Recent evidence has emerged showing that the H3 receptors are also located presynaptically as heteroreceptors on serotonergic, noradrenergic, dopaminergic, cholinergic, and GABAergic (gamma-aminobutyric acid containing) neurons. These H3 receptors have also recently been identified in peripheral tissues such as vascular smooth muscle. Consequently there are many potential therapeutic applications for histamine H3 agonists, antagonists, and inverse agonists. (See: “The Histamine H3 Receptor-A Target for New Drugs”, Leurs, R., and Timmerman, H., (Editors), Elsevier, 1998; Morisset et al., Nature, 2000, 408, 860–864.) A fourth histamine receptor -H4- was recently described by Oda et al., (J. Biol. Chem., 2000, 275, 36781–36786).
The potential use of histamine H3 agonists in sleep/wake and arousal/vigilance disorders is suggested based on animal studies (Lin et al, Br. Res., 1990, 523, 325; Monti et al Eur. J. Pharmacol., 1991, 205, 283). Their use in the treatment of migraine has also been suggested (McLeod et al Abstr. Society Neuroscience, 1996, 22, 2010) based on their ability to inhibit neurogenic inflammation. Other applications could be a protective role in myocardial ischemia and hypertension where blockade of norepinephrine release is beneficial (Imamura et al J. Pharmacol. Expt. Ther., 1994, 271, 1259). It has been suggested that histamine H3 agonists may be beneficial in asthma due to their ability to reduce non-adrenergic non-cholinergic (NANC) neurotransmission in airways and to reduce microvascular leakage (Ichinose et al Eur. J. Pharmacol., 1989, 174, 49).
Several indications for histamine H3 antagonists and inverse agonists have similarly been proposed based on animal pharmacology experiments with known histamine H3 antagonists (e.g. thioperamide). These include dementia, Alzheimer's disease (Panula et al Abstr. Society Neuroscience, 1995, 21, 1977), epilepsy (Yokoyama et al Eur. J. Pharmacol., 1993, 234, 129) narcolepsy, eating disorders (Machidori et al Brain Research 1992, 590, 180), motion sickness, vertigo, attention deficit hyperactivity disorders (ADHD), learning and memory (Barnes et al Abstr. Society Neuroscience, 1993, 19, 1813), schizophrenia (Schlicker et al Naunyn-Schmiedeberg's Arch. Pharmacol., 1996, 353, 290–294); (also see; Stark et al Drugs Future, 1996, 21, 507 and Leurs et al Progress in Drug Research, 1995, 45, 107 and references cited therein). Histamine H3 antagonists, alone or in combination with a histamine H1 antagonist, are reported to be useful for the treatment of upper airway allergic response (U.S. Pat. Nos. 5,217,986; 5,352,707 and 5,869,479). Recently, a histamine H3 antagonist (GT-2331) was identified and is being developed by Gliatech Inc. (Gliatech Inc. Press Release Nov. 5, 1998; Bioworld Today, Mar. 2, 1999) for the treatment of CNS disorders.
As noted, the prior art related to histamine H3 ligands has been comprehensively reviewed (“The Histamine H3 Receptor-A Target for New Drugs”, Leurs, R., and Timmerman, H., (Editors), Elsevier, 1998). Within this reference the medicinal chemistry of histamine H3 agonists and antagonists was reviewed (see Krause et al and Phillips et al respectively). The importance of an imidazole moiety containing only a single substitution in the 4 position was noted together with the deleterious effects of additional substitution on activity. Particularly methylation of the imidazole ring at any of the remaining unsubstituted positions was reported to strongly decrease activity. Additional publications support the hypothesis that an imidazole function is essential for high affinity histamine H3 receptor ligands (See, Ali et al J. Med. Chem., 1999, 42, 903 and Stark et al, Drugs Future, 1996, 21, 507 and references cited therein). However many imidazole containing compounds are substrates for histamine methyl transferase, the major histamine metabolizing enzyme in humans, which leads to shortened half lives and lower bioavailability (See, Rouleau et al J. Pharmacol. Exp. Ther. 1997, 281, 1085). In addition, imidazole containing drugs, via their interaction with the cytochrome P450 monooxygenase system, can result in unfavorable biotransformations due to enzyme induction or enzyme inhibition. (Kapetanovic et al Drug Metab. Dispos. 1984, 12, 560; Sheets et al Drug Metab. Dispos. 1984, 12, 603; Back, et al Br. J. Pharmacol. 1985, 85, 121; Lavrijsen et al Biochem. Pharmacol. 1986, 35, 1867; Drug Saf., 1998, 18, 83). The poor blood brain barrier penetration of earlier histamine H3 receptor ligands may also be associated with the imidazole fragment (Ganellin et al Arch. Pharm. (Weinheim, Ger.) 1998, 331, 395).
More recently, several publications have described histamine H3 ligands that do not contain an imidazole moiety. For example; Ganellin et al Arch. Pharm. (Weinheim, Ger.) 1998, 331, 395; Walczynski et al Arch. Pharm. (Weinheim, Ger.) 1999, 332, 389; Walczynski et al Farmaco 1999, 684; Linney et al J. Med. Chem. 2000, 2362; Tozer and Kalindjian Exp. Opin. Ther. Patents 2000, 10, 1045–1055; U.S. Pat. No. 5,352,707; PCT Application WO99/42458, Aug. 26, 1999; and European Patent Application 0978512, Feb. 9, 2000.
The compounds of the present invention do not contain the imidazole moiety, and its inherent liabilities, and maintain potency at the human H3 receptor. Thus in the present invention receptor binding was determined using the human histamine H3 receptor (See Lovenberg et al Mol. Pharmacol. 1999, 1107). Screening using the human receptor is particularly important for the identification of new therapies for the treatment of human disease. Conventional binding assays for example are determined using rat synaptosomes (Garbarg et al J. Pharmacol. Exp. Ther. 1992, 263, 304), rat cortical membranes (West et al Mol. Pharmacol. 1990, 610), and guinea pig brain (Korte et al Biochem. Biophys. Res. Commun. 1990, 978). Only limited studies have been performed previously using human tissue but these allude to significant differences in the pharmacology of rodent and primate receptors (West et al Eur. J. Pharmacol. 1999, 233).
We now describe a series of aryloxypiperidines with the ability to modulate the activity of the histamine receptor, specifically the H3 receptor, without the inherent problems associated with the presence of an imidazolyl moiety.